While the discussion herein will be, for the most part, directed to bottles made of polyethylene terephthalate (PET) polymers and copolymers, it should be understood that the invention has wide application in thermoplastic articles of various polymers. However, a discussion of the invention as it pertains to such bottles is adequate for a full understanding thereof. Also, it is well known that inherent viscosity (IhV), as well as intrinsic viscosity, are often used in the art as an indication of molecular weight. Hence, IhV, will be used for the most part in describing the present invention herein. IhV is measured at 25.degree. C. using 0.50 gram polymer per 100 mL of a solvent consisting of 60% by weight phenol and 40% by weight tetrachloroethane.
Tg (glass transition temperature) is measured using conventional DSC (differential scanning colorimetry) techniques.
When producing beverage bottles from polymers such as PET, it is important that the polymer be well oriented during stretching. Proper orientation results in uniform material distribution in most areas of the bottle. Some portions of the bottle, such as the threads, support ring, and center region of the base, are not oriented. Attaining the proper orientation is dependant upon the IhV. If the IhV is too low, the bottle will have thin sidewalls which causes a reduction in the shelf-life of the beverage, and the bottle will also expand excessively due to internal pressure.
It should be noted that while the stretching characteristics of PET are highly dependant upon IhV, other factors also have an effect, e.g., the PET's temperature when stretching occurs, the PET's level of copolymerization, moisture content, amount of free volume relaxation, and rate of stretching have an effect on the amount that PET will stretch. However, if these variables are constant for all tests, comparative results are achieved.
Bottles are generally made by first injection molding a preform, which is then blown into a bottle. It is not practical for PET bottle producers to check. bottle preform IhV in their plants. IhV measurements are expensive and can be rather erratic unless the test is very carefully controlled. Therefore, IhV testing is not common in the bottle industry. As a result, most bottle producers often do not learn of IhV problems until bottles are blown (typically 1 to 14 days after molding).
It has now been discovered that IhV can be determined on a relative basis by relating it to the natural stretch ratio (NSR) of articles to be tested. That is, if the NSR of thermoplastic bottles is always determined under the same conditions for different samples, the NSR will indicate the IhV (or intrinsic viscosity or molecular weight). In this way, for example, if an IhV of 0.72 is desired, the NSR of that article can be determined under given conditions (control). A graph for NSR plotted against IhV can be developed. Thermoplastic articles can then be stretched, either unilaterally or bilaterally, under the same conditions as the "control". If this ratio is the same as for the control, it will indicate the same IhV. Or, if this ratio is different, the IhV can be determined by referring to the graph.
It is believed that the present invention may be used by bottle producers to free-blow bottles and then plot bottle volume on statistical process control charts as an indication of IhV.
Free-blowing of PET preforms is a well known technique used to obtain empirical data on the stretching characteristics of a particular PET formulation. Such data are used to design a preform for that formulation which will yield the desired bottle properties. Free-blow involves heating the bottle preform to a temperature above its Tg and then blowing it without a mold such that it is free to expand without restriction until the onset of strain hardening is reached. In PET articles, it will be apparent that the onset of strain hardening has been reached when pearlescence (caused by microcracks) appear. Thus, the free-blow conditions (heating time and blow pressure) are adjusted so that the free-blown bottle will exhibit a slight amount of pearlescence.
Once a particular design is put into commercial use, it is important that the preform IhV be maintained at a target design IhV, or a reasonable variation thereof. If the preform IhV is not near the target IhV, problems will occur during the bottle blowing process. Typically, IhV variations of plus or minus 0.03 dl/g can be tolerated.
A basic problem has existed in the past in consistently heating preforms to the same temperature during each free-blow test to consistently obtain accurate results.